Patent GB-852011-A (GB)
Title: Improvements in or relating to electrical impedance measuring circuits
IPC: G01R17/00
Filed: 1958-01-10
Abstract: 852,011. Measuring bridges; impedance, resistance measurements. WAYNE KERR LABORATORIES Ltd., and CALVERT, R. Jan. 7, 1959 [Jan. 10, 1958], No. 990/58. Class 37 An electrical circuit for measuring unknown impedance comprises a standard impedance, voltage sources for applying voltages of known ratio to the standard and unknown impedances, a high gain amplifier having input and output circuits, means for feeding the difference of currents through the standard and unknown impedances due to the applied voltages into such input circuit, means for coupling the output circuit to feed a current to the input circuit in opposition to the difference of currents to reduce the net input to substantially zero; and an indicator coupled to the output circuit to indicate the amplifier output, which is then proportional to such difference of currents. Fig. 1 shows an A.C. impedance bridge whose ratio arms are the variably tapped opposed secondaries of a supply transformer 10 with a fixed tap connected to the neutral 14, the variable taps being connected through unknown impedance 17 and switchable standard impedances 19, 20 to the input of a high gain amplifier 15 returned to neutral whose output feeds an indicator 23 and is retroactively coupled over auxiliary standard 22 to oppose the unbalance voltage at the input and to reduce it almost to zero. It is shown that for balance, where Yu =admittance of unknown impedance Ys =admittance of standard impedance Ya = admittance of auxiliary impedance Eu =ratio voltage fed to unknown impedance Es =ratio voltage fed to standard impedance Ea =amplifier output voltage so that if Es, Eu are adjusted for coarse balance, the admittance is determined by adding to the known term E S Y S  a correction EaYa derived Eu Eu from the reading of the indicator, which is dependent of the ratio voltage applied to the unknown impedance, and may be standardised by connecting this voltage directly to the indicator. A phase sensitive detector 24 is provided to show the relative phases of current in the unknown impedance and the amplifier output and thus the polarity of the correction factor. The ratio arms may be varied potentiometrically. In a modification (Fig. 2) the input transformer 10 has separate ratio arm windings 25, 26 connected respectively in series with the unknown impedance 17, the input of the amplifier 15 and the output thereof, and in series with the neutral 14 to which the amplifier is returned, the standard impedance 19 and the amplifier input. A phase sensitive detector 31 indicates as before the relative phases of the amplifier output and the unknown impedance current. It is shown that if Zu is the unknown impedance, Zs is the standard impedance, then ZsEu ZsEa so Es Es that the impedance Zu is measured as before by adding a correction term derived from the indication of the amplifier output. The amplifier voltage may alternatively be fed in series with the ratio arm 26. In a further modification (Fig. 3) the supply transformer secondary windings 12, 13 are connected in series with the unknown and standard impedances to separate windings 43, 42 of a composite input transformer 40 whose secondary excites the amplifier input. The output of the latter is retroactively fed back through indicator 45 to a further primary winding 44 whereby the net amplifier input is maintained at approximately zero, so that at balance, Yu =YsEsNs / EuNo + IoNo / EuNo if Io is the amplifier output current and Nu, Ns, No are the turns of the input transformer windings 43, 42, 44. In a further modification (Fig. 4) for complex impedances, the ratio arms of transformer 10 respectively energize the unknown impedance 17 in series with input transformer primary winding 53, and through selectible standard resistances and capacitances 55, 56, 57, 58 in series with input winding 52 which is phase reversedly centre tapped to earth, the input transformer secondary 51 excites the high gain amplifier 15 and the output of the latter is fed back to input transformer winding 54 over auxiliary standard resistance 60. The output is also applied to phase sensitive voltmeters 62, 63 energized with reference voltages in quadrature derived from the supply over preset phase adjuster 66. The tap settings may be inserted manually or by servo-operated switching.